1. Field of the Invention
The invention relates to a method for measuring two-dimensional displacement using conjugate optics, wherein the term, conjugate optics, implies the adoption of the wavefront reconstruction optics and the optics of the telecentric system.
2. Description of the Related Art
A pair of one-dimensional scales in cross-type configuration is ordinarily used to measure two-dimensional displacement. The setup of such a system involves a sequence of alignment steps, one of the most important of which is the alignment for orthogonality, which becomes more difficult as the precision requirement of the system increases. Good technical skills are needed to operate such a system, resulting in a high training cost and significant operator dependence. Recently, planar scales have been designed and developed for the same measuring purpose. This novel instrumentation concept avoids the difficulties of its predecessors.
U.S. Pat. No. 5,204,524 discloses a design of planar scales. The design is rooted in the theory of geometrical optics and intrinsically constrains the precision and resolution of the system.
U.S. Pat. No. 5,424,833 discloses another design of planar scales. In this design, the light beam is diffracted three times by diffraction gratings. The light intensity is thus greatly reduced, and so is the signal to noise ratio. Consequently, in order to build such a system, it is necessary to use better optical components and to ask for stricter assembly tolerances.
U.S. Pat. No. 5,530,543 discloses another design of planar scales that utilizes a single diffraction grating. However, this design compensates for no misalignment, and the output signal is therefore fragile under motion.
The submitted invention proposes a new design of planar scales to resolve the problems mentioned above.
The design uses conjugate optics and is comprised of the following steps. A light source emits a light beam vertically onto a diffraction element; many firstly diffracted beams a regenerated, and at least two of them are reflected along the same optical paths back onto the said diffraction element via corresponding sets of wavefront reconstruction optics; many secondly diffracted beams are generated, of which at least two pairs are respectively superposed by corresponding sets of mirrors and interferometric optics to form interference fringes; two linearly independent displacements of the diffraction element are then obtained by decoding the interference fringes.
The invention has the advantage of providing high tolerances to both the alignment of the optics and the manufacture of the diffraction element. Both the manufacturing and the assembling costs are reduced, and yield is increased.